direct sample rtl-sdr mod

Published April 17, 2017

I’ve started looking back into software defined radio, specifically, the rtl-sdr dongles. I’ve got a few, but haven’t done much with them. I built a simple upconverter a while back, but I built it on a breadboard and it has since been disassembled. I actually bought a ham-it-up upconverter, but it was damaged in shipping and was refunded, so I never actually received it.

Anyway, in researching upconverters, I ran across a mod to use the dongle in direct sample mode. More info can be found here.

Based on this post, I added a DSL transformer that I pulled off an old modem. I used some magnet wire and soldered one side of the transformer to the two ADC pins on the rtl chip. The other side of the transformer was soldered to a bnc connector. A liberal amount of hot glue was applied for some slight strain relief.

Next, I heat shrinked the whole thing in to this blue lumpy monstrosity.

It works. I hooked it up to my 40 meter dipole and plugged it in. I fired up gqrx and set the direct_samp=1 option and it started receiving.

I plugged in my quick 20 meter dipole from yesterday and it apparently sucks. I wasn’t picking up anything which makes me feel better about my bitx. I was thinking it wasn’t working very well, but it appears that it was probably the antenna.